1. Field of the Invention
The present invention relates to a sheet finisher, a sheet finishing method and an image forming apparatus.
2. Description of Related Art
In recent years, a sheet finisher to finish a plurality of sheets into booklet forms of various thicknesses has been required.
In such a sheet finisher, there has been proposed a sheet folding mechanism in a booklet producing apparatus (see, for example, JP Hei 10-279177A), which can accord a folding position by a folding roller with the positions of stitching pins of the booklet (sheet bundle) with high accuracy and can accurately fold the sheets along the positions of the stitching pins of the sheet bundle by moving one folding roller to follow the other folding roller into an opposite direction.
However, the sheet folding mechanism of the booklet producing apparatus (sheet finisher) described in JP Hei 10-279177A has a problem in which it is difficult to apply a proper pressing force for folding according to the thickness of a sheet bundle, although the sheet folding mechanism can accord the positions of the stitching pins of the sheet bundle with the folding position thereof by the folding roller. That is, when the folding pressure of the sheet folding mechanism is adjusted to a thin sheet bundle, the pressing force becomes excessive to a thick sheet bundle, and the fold line of the thick sheet bundle becomes easy to deviate. When the folding pressure of the sheet folding mechanism is adjusted to a thick sheet bundle, the pressing force becomes insufficient for a thin sheet bundle, and the fold line of the thin sheet bundle becomes loose. Thus, the sheet folding mechanism can deal with only a specific thickness.